120 MORPHOLOGY AND EVOLUTION
hinder end of the inferior orbital fissure. The optic foramen is separated from the foramen lacerum anticum, and the spheno-maxillary fissure is long and narrow. The lachrymo-ethmoidal suture is small, the lachrymal hamulus is vestigial, and in some skulls small ossicles lie in the sutures round the ethmoid bone. The naso-lachrymal duct is wide.
The inter-orbital distance is wide as in the Gorilla and Gibbons, whereas it is narrow in the Orang.
The anterior narial aperture is pyriform or ovate, with the narrow end uppermost. It is bounded above by the lower border of the fused nasal bones, and below and laterally by the pre-maxillse. which rise up to meet the nasals and exclude the frontal processes of the maxillae. Owen (374) records the pre-maxillse as not reaching the nasal bones in an immature skull. The anterior nasal spine is rarely present, Keith (100) stating that it was seen by him in five out of forty-three skulls ; but the suture between the pre-maxillae is often seen.
The infra-orbital foramina vary considerably; and a large orifice shows some trace of a division in most skulls; the septum within the main foramen may be complete. Keith (100) states that the partition is vertical in the Gorilla and horizontal in the Chimpanzee, but that is not strictly true. The very first Chimpanzee skull examined by myself had a vertical septum in the infra-orbital foramen. Additional foramina may be present alongside the main orifice; and foramina for the zygomatico-facial vessels and nerve are always present on the flat malar bones.
The nasal bones fuse at or soon after birth to form a quadrilateral bone, and the lower borders of the conjoined